1. Field of the Invention
The present invention relates to a transmission belt such as a toothed belt, a V belt, and a V-ribbed belt, which transmit a drive force of an engine of an automobile to an accessory device, for example.
2. Description of the Related Art
In general, a toothed belt is provided with a belt body, cords, and a facing fabric. The belt body has teeth, receiving a load drive, and is made from rubber. The cords serve as tension bearing members, and are embedded along a longitudinal direction of the belt body. The facing fabric covers the tooth surfaces of the belt body for preventing wear.
When a toothed belt is used as a timing belt of an automobile, it is necessary that it be able to withstand use in a high temperature and high load environment. Therefore, various improvements in the materials used for toothed belts and the method of bonding of the materials have been experimented with. Specifically, a hydrogenated nitrile rubber, superior in heat resistance, has been used for the stock rubber of the belt body in place of conventional chloroprene rubber, high strength aramid fiber has been used for the cords in place of glass fiber, and a fabric comprised of aramid fiber, superior in heat resistance, has been used for the facing fabric in place of a Nylon-6, 6 fabric.
The belt body is subjected to a vulcanization process for creating three-dimensional structures by heating or otherwise treating a rubber composition comprised of the above stock rubber plus sulfur or an organic peroxide or other vulcanizing agent or plasticizer etc., whereby the elasticity, tensile strength, and dimensional stability can be improved. Further, the cords are treated for bonding by coating and drying a rubber cement or resorcinol formaldehyde latex (hereinafter referred to as “RFL”), and are then embedded in the unvulcanized belt body. At the same time as the vulcanizing molding of the belt body, the adhesive is reacted due to the heat, causing the cords to be strongly bonded to the rubber of the belt body.
In recent years, there has been research and development on poly-p-phenylene benzobisoxazole fiber (hereinafter referred to as “PBO fiber”) more superior in heat resistance, flame retardance, and dimensional stability than aramid fiber. PBO fiber, however, has an extremely high crystallinity and a close-knit structure, so is poor in bondability with rubber cement or RFL adhesive. Accordingly, there is the problem that even if the conventional treatment for bonding is applied to the cords, they will not bond with the rubber of the belt body. Namely, in a toothed belt receiving a large repeating load, there is the problem in which, if the bondability of the cords is poor, interfacial breakage will occur between the two and lead to early breakage of the toothed belt.